Test: cover audio format variants in the mock-game stress suite

mock_game_test now launches coop_mock_game at 44100/48000/96000 (PCM + float)
and asserts the hook measures each variant's rate through the full inject path
(+ non-silent capture) -- the "all audio variants work" part of the suite.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -214,10 +214,12 @@ ctest --test-dir build -C Debug --output-on-failure
   hook's shared video texture, and **decodes the frame number out of the captured pixels**
   to assert the mirror sees a *monotonic, advancing* sequence for both backends (the bar
   for no dropped / stale / out-of-order frames — what the DX12 rotating-backbuffer bug
-  broke). It then injects with audio + video, checks both stream, cycles the audio
-  subsystem off/on (hook/unhook stress), and confirms the game never crashes and capture
-  resumes. This suite drove out five real audio races (see Lessons learned). Skips cleanly
-  without a D3D11 device.
+  broke). It launches the game at several **audio formats** (44100/48000/96000, PCM +
+  float) and asserts the hook measures each one's rate through the full inject path, then
+  injects with audio + video, checks both stream, cycles the audio subsystem off/on
+  (hook/unhook stress), and confirms the game never crashes and capture resumes. This
+  suite drove out five real audio races (see Lessons learned). Skips cleanly without a
+  D3D11 device.
 
 ### Debugging the hooks against a real game